1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a laser printer, an electrophotographic facsimile or the like, and more particularly, to an image forming apparatus provided with a guiding member which is disposed on a downstream side of a transferring device in a feed direction.
2. Description of the Prior Art
Recently, image forming apparatus are getting smaller in size for personal use. Therefore, it is difficult to provide a particular separating device such as a separating discharger for separating a receiving substrate such as a sheet material from an image bearing member after transfer. It may otherwise be provided with a separating belt, but an image cannot be formed at a portion where the separating belt is positioned, so that the copy produced lacks a part of image and it is not desirable for copy quality. On the other hand, as the image bearing member such as a photosensitive drum gets small, the sheet material can be detached from the photosensitive drum without any particular separating devices. In other words, if the photosensitive drum has a relatively big curvature, the sheet material is detached from the photosensitive drum by virtue of its stiffness and its own weight.
However, there are many charges on a rear surface of the sheet material which is detached from the photosensitive drum by itself. As a result, a downstream part of the sheet material after being detached from the photosensitive drum waves in the feed direction during conveyance because of the charges on the rear surface of the sheet material. The waves are transmitted from the downstream part of the sheet material to an upstream part thereof onto which a toner image on the photosensitive drum is being transferred, which causes inferior transfer. Therefore, the charges on the rear surface of the sheet material have to be removed as soon as possible after detachment from the photosensitive drum.
However, if a conductive material is provided in close vicinity of the transferring device such as a transfer discharger, charges from the transfer discharger flows as an electric current through the conductive material to the ground, which also causes inferior transfer due to lack of charges for transfer. And even if the conductive material is disposed so that the sheet material may come into contact with the conductive material after detachment from the photosensitive drum, the charges on the rear surface of the sheet material flows as an electric current through the conductive material rapidly, which causes crumbling of the toner image on the sheet material due to the shock.
There is proposed an image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 126571/1984. This image forming apparatus comprises a transfer discharger, an insulating member and a charge removing member which are provided in this order in a sheet feed direction. The charge removing member is provided for removing the charges on the rear surface of the sheet material, after the sheet material is detached from the photosensitive drum by virtue of its stiffness and its own weight. And the insulating member is provided between the transfer discharger and the charge removing member in order that the charges from the transfer discharger may not flow as an electric current to the charge removing member.
However, the conventional apparatus has a following problem. That is, the charge removing member is helpful for removing the charges on the rear surface of the sheet material as described hereinabove, but the insulating member is provided on the upstream side of the charge removing member, which diminishes the charge removing action of the charge removing member and delays timing of the charge removing. More concretely, an area of the charge removing action performed by the charge removing member is not limited to an outer edge of the charge removing member, but extends over the circumference. However, the insulating member disposed on the upstream side of the charge removing member cuts off the charge removing action. On the other hand, particularly, when the receiving substrate is not a plain paper but a sheet used for an Over Head Projector (OHP), said OHP sheet holds many charges, therefore the charges have to be removed certainly and effectively.
However, in the conventional apparatus, the charge removing action cannot spread over the insulating member, thus the charges cannot be removed immediately and sufficiently. As a result, some of the problems discussed hereinabove, that is, the waves of the sheet material accompanied with the inferior transfer cannot be prevented.
Moreover, the insulating member holds many electrostatic charges which comes from the transfer discharger and are generated as the sheet materials pass many times. The electrostatic charges on the insulating member used for a guiding member causes not only poor conveyance of the sheet material but also adhesion of toners. The poor conveyance of the sheet material causes crumbling of toner image on the downstream part of the sheet material which is detached from the photosensitive material, and also causes inferior transfer on the upstream part of the sheet material onto which the toner image on the photosensitive drum is being transferred, and further causes a sheet jam.
The toners adhesion causes stains of the sheet material which is conveyed on the insulating member used for the guiding member. More particularly, if the transfer discharger is the type which a part of an opening of the transfer discharger is covered with a part of the guiding member, the problem is more serious. That is, the guiding member covers the part of the discharger's opening in order to regulate a breadth of discharging area and to prevent a tip of the sheet material from entering inside of the transfer discharger, but the guiding member is charged by the transfer discharger because of its insulation. The guiding member covering the part of said opening is made of insulating material in order to prevent the inferior transfer due to the electric current flow. Thus, the charged guiding member attracts floating toners scattered in the neighborhood of the photosensitive drum. And the toners adhere to the surface of the guiding member, thus the tip and the rear surface of the sheet material becomes dirty when the sheet material is guided by the guiding member.